22 Neogene Tectonic Geomorphology and Landscape Evolution in the Intermountain West

Tuesday, 20 May 2025
8:00 AM - 9:45 AM
Cascade C

FAULT BLOCK, TECTONIC DETACHMENT, OR MEGA-LANDSLIDE? EAST TRAVERSE MOUNTAINS, UTAH

Paper number: 22-4
Program: Theme Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

The structure and origin of the east Traverse Mountains (eTM) of central Utah, which trend perpendicular to the Wasatch fault, have been debated for over a century. The eTM have historically been explained as a normal fault-bounded salient or as the hanging wall of a detachment. Our research uncovered multiple lines of evidence that the eTM are not a tectonic feature, but are instead a ~75 to 150 km3 mega-landslide deposit that catastrophically slid 16 km from the upper reaches of the Little Cottonwood stock to its present location in the late Miocene. There is pervasive brecciation of quartzites, limestones, volcanic strata, and dikes in the block that are broken into fragments <10 cm across. Fracture surfaces are locally polished and have slickenlines. Pseudotachylyte and cataclasite near the slide’s inferred base consistently dip shallowly to the southwest parallel to slickenline orientations, and they form a shear zone that was offset by later slip on the steeper Fort Canyon normal fault. A swarm of andesitic dikes and a separate set of Mo-mineralized pebble dikes within the landslide’s inferred source region appear to correlate with similar dike swarms in the eTM based on dike orientations, mineralogy, geochemistry, and age, suggesting they have been offset 16 km to the southwest. Distinctive normal faults and sedimentary strata now found in the eTM are also displaced. Cooling ages (~6 Ma) for the Oligocene granodiorite in the landslide source are anomalously young. Near the western flank of the megabreccia, a distinctive Miocene conglomerate within a succession of fine-grained lacustrine sediments in the Salt Lake Formation appears to be the sedimentary response to the landslide. U-Pb ages of post-slide opal and a pre-slide tuff constrain the age of the slide to be about 6.5 Ma. We propose that, as a result of progressive normal faulting, the mountain front had become over-steepened and gravitationally failed, and the roof of the Little Cottonwood stock slid 16 km to the southwest. Failure may have started in the pluton’s roof, which was hydrothermally altered by multiphase intrusions, and includes the weak and shaley Doughnut Formation. Understanding mega-landslides such as this is crucial for assessing potential hazards in extensional basins and for distinguishing large landslides from tectonic features.

Geological Society of America Abstracts with Programs. Vol. 57, No. 5

doi: 10.1130/abs/2025RM-410192

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.